In the process of producing semiconductor integrated circuits, an etching process using low-pressure plasma or other plasma is performed to form micro-sized holes or grooves in a semiconductor substrate, such as a silicon wafer. In normal etching procedures, the area where no hole or groove should be created is initially masked with a resist layer on the substrate before the etching process. During the etching process, the areas where no resist mask is present are selectively etched. After this process is completed, the resist layer is removed to obtain a substrate having holes or grooves (hereinafter referred to as “holes or the like”).
During the etching process, not only the areas where no resist mask is present, but also the resist layer in the masked areas may be etched to a decreased thickness, or the etched material may be deposited on the resist layer in the masked areas to increase the thickness of the resist layer. A temporal change in the thickness of the resist layer is thus monitored in real time during the etching process, and the obtained result is used as a criterion to determine whether the etching process is normally proceeding.
In one of the methods of monitoring the temporal change in the thickness of the resist layer, a light source with a broad wavelength range (a white light source) is used. In this method, a white light is cast onto the substrate whose surface is masked with a resist layer. The light reflected by the surface of the resist layer, and the light passing through the resist layer and reflected by the surface of the substrate are made to interfere in an interference optical system. The obtained interference light is wavelength-dispersed by a light-dispersing element, and detected by a CCD line sensor or the like to obtain wavelength-intensity characteristics.
When the white light is used, however, the light entering each sensor after the wavelength dispersion has a low intensity. Thus, it takes time to measure the thickness of the resist layer and the thickness of the resist layer cannot be measured in real time.
To solve the problem, the methods of performing real-time measurement by using a laser light source or a super luminescent diode (SLD) light source have been proposed. Patent Literature 1 discloses the method of casting a laser beam onto a substrate whose surface is masked with a resist layer to obtain an interference light in a similar manner to the previous manner, and monitoring a temporal change in the intensity of the interference light. Along with the progress of etching, a local maximum value and a local minimum value appear in the intensity of the interference light at each time when the layer thickness changes by one-quarter of the wavelength of the laser beam cast onto the substrate. Accordingly, in this method, the temporal change in the intensity of the interference light is monitored to measure the thickness of the resist layer or the depth of the hole or the like.